Single cells in the cat striate cortex are more selective for the spatial frequency of sinewave grating stimuli than are cells of the retina or lateral geniculate nucleus. We have explored the possibility that this enhancement of selectivity results from spatial-frequency-selective inhibition. Stimulation with two superimposed gratings, one to excite the cell and one to prove for inhibition, revealed spatial frequency-dependent response suppression in 74% of the total population studied. Suppression was slightly more prevalent in simple cells (80%) than in complex cells (68%). In 93% of the cases where suppression was found, its tuning was complementary to excitatory spatial frequency tuning, and the strongest suppression was usually found where the excitatory tuning function approached zero imp./sec. Characteristics of the phenomenon were independent of cortical layers. We conclude that organized inhibitory mechanisms serve to refine the spatial frequency bandpass of striate cortical cells. This provides evidence for another degree of nonlinearity in the organization of cortical receptive fields and supports the hypothesis that a fundamental function of the visual cortex is image dissection in the domain of spatial frequency.